Phonograph recording head



MayA 18, 1948. A L, THOMPSON 2,441,651

PHONOGRAPH RECORDING HEAD Filed Dec. '27, 1945 INVENTQR. wcm/v Ffm/PSOMATTORNEY tom than the top gaps.

Patented May 18, 1948 UNITED STATES PATENT OFFICE PHoNocRAPH RECORDINGHEAD Lneohi Thompson, New Haven,` Conn., assgnor to The SoundscriberCorporation, New Haven,

Conn., a corporation of Connecticut Application December 27, 1945,Serial No. 637,37

armature is pivotally mounted to vibrate with respect to aiixed'solenoid which is electrically energized in accordance with thesound vibrations to be recorded.

' The present invention is an improvement over that disclosed inapplicants prior Patent No. 2,222,342, issued November 19, 1940, andentitled Electromagnetic sound recorder. Y

In the present invention the sound recording head has a pair of polepieces which embrace a two-piece armature formed of two verticallyaligned sections, the upper section being stationary and surrounded by ashort energizing coil, and the lower section being movable, and carryingthe recording stylus-said armature sections being spaced from each otherby a small air gap.

One object of the present invention is to provide a recording head oftheabove nature in which the lower stylus-carrying section of the armatureis provided with an integral horizontal rearwardly-extending bearingshaft which is resiliently secured to the head frame, the stylus beingfree t-o oscillate within a rubber-like block which dampens thevibrations thereof.

A further object is to provide a recording head of the above naturehaving side lgaps between the upper stationary armature section and thepole pieces, which gaps are lled with non-magnetic material, and arelarger than the corresponding side air gaps between the movable bottomarmature and the pole pieces, so as to produce a greater magnetic fluxthrough the bot- A further object of this invention is to provide animproved electromagnetic sound recording head of the above nature-whichwill be relatively simple in construction, inexpensive to manufacture,compact, ornamental in appearance, and very eicient and durable `in use.

With these and other objects 'in View, which will appear as thisdescription proceeds, there has been illustrated on 'the accompanyingdrawing one form in which the'invention may conveniently be embodied inpractice.

In the drawing,

Fig. 1 is a schematic view of a sound recording apparatus embodying theinvention.

Fig. 2 is a longitudinal sectional view recording head'of suchapparatus.

of the Fig. 3 is av transverse .sectional View, taken along the line 3-3of Fig. 2, looking in the direction of the arrows. l

Fig. 4 is a transverse sectional View taken along the line 4-4 of Fig.2, looking in the direction of the arrows. y

Fig. 5 is a bottom view of the recording head with the base plate andcasing removed to show the interior parts.

Referring now to the drawing in which like E reference numerals ydenotecorresponding parts throughout the several views, the numeral I0indicates a vertical lower langular movable armature sectionof magneticmaterial, said lower armature section I0 having a horizontalrearwardly-extending dat section II, the ends of which have a pair ofhorizontal opposed stiii' torsional pins I2, I3 provided with a pair offront and rear squared blocks I4, I5 integral therewith. The blocks I4,I5 are rigidly secured, as by soldering, in a non-magnetic ellipticalring plate I6, preferably of brass.

The movable armature section IIJ is provided with a square ldependingleg I1 having a hollow socket I8 for receiving the shank of an inclineddiamond-tipped recording stylus I9 which is adapted to indent soundgrooves 20 in a disk record 2| mounted on a turntable 22.

The lower movable armature section I0 eooperates with an upperstationary flat armature section 23 spaced vertically therefrom by asmall air gap 24,k The armature sections I0 and 23 are embraced by aU-shaped magnetincluding a pair of fixed L-shaped magnetic pole pieces25, 26, between the front of which is a rectangular upper aperture 21enclosing a relatively short solenoid coil 28 which surrounds the upperarmature section 23. Above the coil 28, the pole pieces 25, 26 haveinturned arms 29, 30, and

below said coil inturned arms 3 I, 32 are provided.

The coil 28 is energized from an amplifier 33 which is adapted toreceive audio-frequency currents from a microphone 34. The ring plate I6is secured to the lower pole piece arms 3l, 32 by a pair of screws 35,35.

vThe upper fixed armature section 23 is spaced from the pole piece arms29, 30 by a pair of relatively largeside gaps 31, 38 ywhich are lilledby a pair of non-magnetic spacer blocks 39, 40 asshown in Figs. 3 and 4,said blocks being soldered in position. The lower movable armaturesection I0 is spaced from the pole piece sections 3l, 32 by a pair' ofrelatively small side open air gaps 4I, 42. By means of thisconstruction, kthe steady magnetic flux passing through the mov,- ablestylus-carrying armaturesection I0 will be,

3 greatly enhanced, causing the sensitivity of the recording head to be,at the maximum and resulting in a superior performance thereof.

It has been found that the sensitivity of the recording head will reacha maximum when the upper side gaps 31, 33 are about fty per cent greaterthan the lower side air gaps 4I, 42. If the upper side gapsrare madelarger `than this, however; the sensitivity of the recording head willstart to diminish because of the introduction of too much reluctance tothe flow of the alternating magnetic flux. It has been'found'that if theupper side gaps 31, 38 are made larger with respect to the lower'gaps43I, -4'2 lmore off'the steady flux due to the permanent magnetsv willflow through the lower gaps 4 I, 42, than throug said upper gaps. l

It has also been found that there is an optimum relationship between theupper gaps 3l, v38"a"ndY the lower gaps 4I, 42 for maximum eilciency ofthe recording ihead. Thus,if the upperfgaps .31, 38 are made too large,however, a criti'calpoint will' be vreached where the reluctance of4said upper Vgaps Awill reduce the uX `in the entire vmagnetic circuit.This is illustrated bythe `following u tab-le which :give the results oftests made with a recording "head embodying Vthe present invention andusing signals at 400 cycles. The :size of `the upper :and lower :gapsisexpress'ed in inches-and the :power offsensitivityis expressed indecibels p Upper gaps Lower 'gaps Povfsgnsl' Inches Inches b. 007 '-2O10 007 -2 012 007 4 .'014 007 0 027 007 It will be noted from thistable Ythat the peak sensitivity is obtained when the upper `gaps are.0'1'2 inch, `and that for larger upper'gaps the sensitivity levels ofidue to the increase in the reluctance 'of-'the magnetic circuit.

The recording head is provided with a protectiveca'sing143 withinwhich'is secured a'lrear block 44' of permanently magnetized material,said bl'ockFbeing'held -withi-n the sides foi fthe L--shaped pole pieces25, 2'6 'by a horizontal elongated sc'rewwtl In order to dampenundesired resonant vibrations yof the movable armature section I0,provision is Amade of a thick rectangular bloc-kl of soft yiel'dableresilient rubber-"like material havingfa square central aperture 741which is fitted tightly about the depending leg I? of the l'ower movablearmature It and preferably cemented thereto. The block 46 is held in:place in a rectangular recess 'il'of -a non-magnetic base'vplate 49,-detachably secured `to the "pole piecesf25,- '-2`B by'means of-smallscrews 59, 5l. The block 45 hasf'afpairofside bottom ribs52, 53 whichengage the base plate-"49 and serveto space the central part voi ssaidblock therefrom.

, Y Y It will -be understood that the damplnglblock Y 4 such materialsis due to the fact that they are somewhat plastic and have a highinternal molecular resistance to flow-requiring work to alter theirshapes.

The coil 28 is connected to the amplier 33 by means of a pair ofconductors 54, 55 which pass out of the casing in a groove 55 in the topof the base plate lIMJ.

The recording head is connected by a flexible leaf spring 51 to ahorizontal recording arm 58 which is power-driven by means of a verticalshaft 59 .passing through a chassis 60 above which is located theturntable 22 which is driven by a shaft 965|. Y

' Inor'derto 'supply power to operate the amplifier-33 'and theA'turntable motor (not shown), provision isrmade ora-power plug 62connected by a pair of conductors 63, 64 to said amplifier. Themicrophone 34 is likewise connected to the amplier 33 by means of a pairof conductors 65, 66.

'Operation In operation, a Iluxof steady magneticlines of force willpass across'ft-he upper side rgaps 31,138, the lower air gaps 4I, A4-2,and through the upper and lower armature sections 2-8 4and I8,respectively-the magnetic circuit .being completed through the polepieces 25, 26 and theY magnet block 44. The stylus I9 will becau-sed tovibrate laterally; around the axis vof the pins I2, I3 in responseY tothe electrical -oscillation-s Ysupplied to the coil 28 from.themicrophone .34 through the ainplier.

One advantage of the present inventionis that the vend lblocks I4., I5connected to the torsional pins I2, I3 ofthe movable armature sectionIil are rigidly mounted in the -ringplate I6, which inturn is rigidlysecured to the pole pieces .25, 25. This constructioncausesthe lowerarmature section-I1 to be ineXi-ble vertically, and insures that the airgap 24 between the upper and lowerarmature sections '23 yand I04will-notnbe diminished vby the pressure of the stylus 'I9 upon .thedisk record 2'I.

A still further advantage vdueto the novel stiiT relatively inflexiblemounting -cf the movable armature ill inthe recording head, 4is thatextremely accurate :groove spacing will be maintained throughouttheentire recording. This will prevent undesired displacements oftheerecordystylus -under the impact of external shocks which frequentlyoccur vwhen-flexibly mounted recording styluses areA employed.

A'still furtheradvantagefof the rigid mounting of 'movable arma-tureIliiis that it'results in a high natural-period of vibration `of saidarmature. ThisV assists vin -accentuating the higher frequencies of thesound waves being recorded and-in attenuating `the .lower lfrequencies'thereof, the :desirability of which is indicated rin the prior PatentN'o.' 2,239,042 to Kleber et al. of April '22, 1941, 4'entitled4 Wa-ve'recording and reproduction. This :stiff `construction also avoids overcutting `which sometimes occurs when recording vals'f'slow groovespeeds, by vthe apparatus:disclosed'in my prior ReissuerPatent No,f22,.83,issued VSeptember 22,` 1942 (original No; '2200;865 `'oflli/layle, 1940:), entitled Apparatus i or recording sound on `ithinfdislns.

While there has been disclosed this specification one form `inxwhich.the invention may :be embodied, itis to 'be understoodthat this 'formiseshown for the purpose oiillustrationfonly, .and that the invention:ls'notto :beliinite'd'.to the-speciiic disclosure, but'criayche`zxn'cziified` and vembodied in various other forms without departingfrom its spirit. In short, the invention includes all the modicationsand embodiments coming within the scope of the following claims.

Having thus fully described the invention, what is claimed as new, andfor which it is desired to secure Letters Patent, is:

1. Inal phonographic sound recording head, a movable lowerstylus-holding armature section 0f magnetic material, a stationary upperarmature section cf similar material arranged in substantial alignmentwith said movable armature section leaving a horizontal air gaptherebetween, a permanent magnet embracing both of said armaturesections leaving two pairs oi upper and lower vertical side gapstherebetween, the upper side gaps being substantially larger than thelower side gaps to produce a greater magnetic iiux through the lowermovable armature section than the magnetic flux through the upper fixedarmature section.

2. The invention defined in claim 1, in which said upper side gaps arefilled with Vertical blocks of non-magnetic material.

3. In a phonographic sound recording head, a movable lowerstylus-holding armature section of magnetic material having a pair offront and rear cylindrical torsion rods integral therewith, a, pair ofsquared end blocks rigidly secured to said rods, a stationary upperamature section ofk similar material arranged in substantial alignmentwith said movable amature section leav.- f

ing a horizontal air gap therebetween, a permanent magnet embracing bothof said armature sections leaving two pairs of upper and lower Vverticalside gaps therebetween, the upper side gaps being substantially largerthan the lower side gapsto produce a greater magnetic iiux through thelower movable armature section that the magnetic flux through the upperxed armature section, and an open ring plate secured to said kmagnet andrigidly secured to said end blocks. LINCOLN THOMPSON.

REFERENCES CITED The following references are of record inthe iile ofthis patent:

